This invention relates to fluorescent lamps and more particularly to highly loaded electrodeless fluorescent lamps. By highly loaded is meant lamps having a discharge current of equal to or greater than 2 amperes.
Electrodeless lamps, such as those shown in U.S. Pat. No. 5,834,905, employ a hollow glass envelope containing mercury vapor and a buffer gas and having a phosphor coating on the inside surface of the glass body. The phosphor is a substantially homogeneous mixture of usually three materials having emission spectra in different parts of the visible spectrum and blended to emit white light. In preferred forms of this lamp the blend includes three phosphors, namely, red emitting Y2O3:Eu3+, green emitting LaPO4:Ce3+,Tb3+ and blue emitting BaMgAl10O17:Eu2+. These lamps are designed for extremely long life, i.e., in the neighborhood of 100,000 hours. The long life of these lamps has given rise to problems involving a color shift in the emitted spectra. It has been determined that this color shift is caused by degradation of some of the lamp phosphors during life, the degradation probably being caused by the long exposure to at least one of several wavelengths of ultraviolet radiation generated during operation of the lamp.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art and to improve the performance and consistent color output of such lamps.
These objects are accomplished, in one aspect of the invention, by the provision of a highly loaded fluorescent lamp comprising a hollow, translucent glass body containing a medium capable of generating at least several wavelengths of UV radiation. A plurality of phosphors is disposed on the inside surface of the glass body, the plurality of phosphors emitting visible radiation upon exposure to the UV radiation. At least one of the plurality of phosphors is subject to degradation upon long-term exposure to one of the at least one of several wavelengths of UV radiation. The at least one of the plurality of phosphors subject to degradation is installed adjacent the inside surface of the glass body to form a first layer; and, the remainder of the plurality of phosphors are disposed on the first layer to form a second layer, the second layer not being subject to long-term degradation upon exposure to the UV radiation.
The deposition of the more stable phosphor on top of the least stable phosphor provides adequate protection of the least stable phosphor during the life of the lamp, allowing the lamp to more nearly provide its designed color output during its long life.